The present invention relates to devices and methods for offset and similar printing systems.
More particularly, the present invention relates to devices and methods for screening information from a continuous tone original to produce a halftone image on a photosensitive layer.
In particular, the invention relates to a novel screen, photomechanically or electronically generated, for the preparation of offset or similar printing plates to be used in offset or similar printing of halftone copies of continuous tone originals.
There exists a wide variety of photomechanical screens. Their purpose is to break up the continuous tone information of the original into discrete dots, whose size is related to the optical density of the original. The resultant film, upon processing, produces a "dot" image of the original.
In the prior art systems all the dots, which are small enough for the reconstruct into a coherent image are of equal opaque optical density while their relative sizes produce the impression of various grey values. The practical resolution of these screens, and those generated electronically (e.g. via an Electronic Dot Generator Scanner) is usually about 100-200 dots/inch.
In the past, the standard screen utilised an array of symmetrical round or square dots. The details of the original reproduced through such a screen are determined by the arbitrary geometrical centres of the dots in the screen. Further, in the resulting halftone, until 50% dot density one has black dots on a transparent background. An abrupt jump in the tonal scale occurs at 50%, where the four corners of the square dot join at 50%. This discontinuity prevents smooth midtone transitions.
To overcome the latter limitation, an elliptical, diamond, multi-dot or star-like dot pattern is mainly used in modern offset printing. These dot shapes allow a smoother tonal gradation than the square dot, in the 50% area, Two opposite corners of e.g., the diamond shaped dot, join the adjacent dots first at about 40%, while the other two remaining corners join adjacent dots, near 60%. Since the dots join in two steps, a smoother tonal transformation is achieved, This strategy, of having adjacent dots join in multiple steps, is used, for example, in the Double Dot Policrom Screen and the Triplet Dot HRS Beta Screen. The result is smoother, but far from ideal, as the abrupt jump in tonal scales is only attenuated to some degree. Aside from these jumps the image remains discontinuous in the X and Y direction because of the spacing between the dots in both directions which becomes more apparent with a lower screen ruling.
In U.S. Pat. No. 4,768,101 there is disclosed a method of generating a half-tone representation of an image from digital data defining the colour content of pixels arranged in a series of substantially parallel input scan lines.
U.S. Pat. No. 4,700,235 discloses a method and apparatus for producing half-tone printing forms with screens having arbitrary screen angles and screen width.
U.S. Pat. No. 4,833,546 discloses a photomechanical apparatus adapted to print a half-tone picture corresponding to an original continuous tone picture on the basis of tonal information signals obtained by photoelectrically scanning the original continuous tone picture.
U.S. Pat. No. 4,547,812 discloses a method and apparatus for forming high resolution half-tone images.
U.S. Pat. No. 4,543,613 discloses a method for producing half-tone dots in a half-tone plate recording apparatus.
As will be realised none of said patents teach or suggest the device and method of the present invention.